1. Field of the Invention
Embodiments of the present invention relate to the field of power circuits. More particularly, embodiments of the present invention relate generally to power circuits that generate low voltage power in standby conditions.
2. Related Art
Electronic devices are built to accept control signals even in a standby condition. That is, although the electronic device is turned off, the device is placed in standby condition. From the standby condition, the electronic device consumes power while waiting to accept a control signal that, for example, starts up a switching power supply circuit. The power is consumed by the logic waiting for the control signal, displays (e.g., light emitting diode [LED] monitors), etc.
Unfortunately, the standby energy consumption of the electronic device may be as high as ten percent of the total energy consumption of the device, especially when the electronic device spends a majority of its operating lifetime in a standby condition. The rate at which energy is transferred or converted is power, wherein the unit of power is the watt (1 watt is the rate of energy transfer of 1 joule per second). As such, the electronic device consumes a significant amount of power just waiting for a control signal when in the standby condition. For example, any electronic device may consume upwards of three to twelve watts of power in the standby condition.
This standby power consumption can quickly add up for a particular household, for example, when considering the number of electronic devices that can be in standby condition at one time. For instance, electronic devices that consume power in a standby condition include, but are not limited to, televisions, DVD players, washer and dryers, appliances, garage door openers, personal computers, printers, unloaded chargers, etc.
Prior Art FIG. 1 is a diagram of a power circuit 100 that generates a high voltage output in tandem with a low voltage output using a resistive voltage drop and a shunt regulator. A high alternating current (AC) voltage (e.g., 110 or 220 volts AC) is applied at a connector 110. The diode bridge 120 rectifies the AC voltage from the connector 110 to a direct current (DC) voltage. The capacitor 130 filters the DC voltage to reduce the ripple to an acceptable level. The high DC voltage is presented on connector 140.
The power circuit 100 generates a low voltage from a high DC voltage through the use of resistor 150. This low voltage is stabilized through a zener diode 160. The low voltage is presented to connector 190. However, the use of a resistor to drop the voltage results in a high power loss through the resistor 150. This power loss is greater than the recommendations issued by the regulatory agencies in the United States and Europe, for example. For instance, the power consumed by the power circuit 100 is greater than 1 watt when the connector 190 is loaded.
Prior Art FIG. 2 is a diagram of a power circuit 200 that generates a high voltage output in tandem with a low voltage output using a resistive voltage drop and a shunt regulator. A high AC voltage (e.g., 110 or 220 volts AC) is applied at a connector 210. The diode bridge 220 rectifies the AC voltage from the connector 210 to a DC voltage. The capacitor 230 filters the DC voltage to reduce the ripple to an acceptable level. The high DC voltage is presented on connector 240.
The power circuit 200 generates a low voltage from a high DC voltage through the use of resistor 250. This low voltage is stabilized through a zener diode 260. In addition, a linear regulator integrated circuit 270 is included to improve the regulation of the low voltage power signal by stabilizing the low voltage power signal presented to connector 290. However, the power circuit 200 suffers from the same power consumption inefficiency as the power circuit 100 of Prior Art FIG. 1. Specifically, the power dissipated through resistor 250 is too great, and the power consumed by the power circuit 200 is greater than 1 watt when the connector 290 is loaded.
Prior Art FIG. 3 is a diagram of a power circuit 300 that is capable of generating a high power output in tandem with a low power output using a transformer and a series linear regulator. A high AC voltage (e.g., 110 or 220 volts AC) is applied at a connector 310. The diode bridge 320 rectifies the AC voltage from the connector 310 to a DC voltage. The capacitor 330 filters the DC voltage to reduce the ripple to an acceptable level. The high DC voltage is presented on connector 340.
The power circuit 300 generates low voltage by employing a step down transformer 350. The diode bridge 360 rectifies the AC voltage from the step down transformer 350. A linear regulator 370 is used to stabilize the voltage from the diode bridge 360 that is presented to the connector 390. The power circuit 300 is more efficient than the power circuits 100 and 200 of FIGS. 1 and 2, respectively. However, the power circuit 300 is more expensive to implement since a step-down transformer 350 is used to lower the voltage down to an acceptable level. In addition, the use of the step-down transformer 350 is bulky and may unnecessarily increase the size of the power circuit 300.
Prior Art FIG. 4 is a diagram of a conventional power circuit 400 of a switching power supply that is capable of generating a low power output through a resistive voltage drop. A high AC voltage (e.g., 110 or 220 volts AC) is applied at a connector 410. The diode bridge 420 rectifies the AC voltage from the connector 410 to a DC voltage. The capacitor 430 filters the DC voltage to reduce the ripple to an acceptable level. The high DC voltage is presented to a transformer 440 that steps up or down the DC voltage as applied to the system 450. Various stepped up or down voltages (e.g., Va, Vb, and Vc) can be applied to the system 450.
The power circuit 400 generates low voltage by employing a step down secondary winding 480 on transformer 440. As such, the isolation transformer 440 is always on. In addition, the low voltage is applied to a microcontroller 490 on the low side of the circuit 400. The use of the auxiliary transformer secondary winding 480 is expensive and bulky, thereby introducing the inefficiencies of the power circuit 300 of FIG. 3.
Moreover, the power circuit 400 includes a power management (PWM) controller 460 that adjusts the duty factor of the pulses presented on the gate of transistor 470. In order to start the power circuit 400 to apply power to the system 450, the resistor 475 injects a current necessary to get the PWM controller 460 to turn on. However, most of the voltage is dropped through the resistor 475. This resistor 475 drops the high voltage (e.g., 100 to 400 volts) from the output of the diode bridge 420 to a lower value (e.g., 6-36 volts). Unfortunately, the use of a resistor to drop the voltage results in a high power loss through the resistor 475. As such, the power circuit 400 suffers from the same inefficiencies as the power circuit 100 and 200 of Prior Art FIGS. 1 and 2, respectively.
During the operation of the power circuit 400 in presenting high voltage to the system 450 a larger current is needed in order to drive the gate of transistor 470. This additional current can be achieved through an auxiliary winding of the transformer 480, diode 482, and capacitor 484 to rectify and filter the AC voltage generated by the auxiliary winding of transformer 480.
Although most of the energy required to keep PWM controller 460 running is sourced from the auxiliary transformer 480, the resistor 475 still dissipates a significant amount of power. To reduce this loss through resistor 475, the power circuit 400 can be improved by disconnecting resistor 475 after the power supply in power circuit 400 is started. This is possible, for example by adding a high voltage PNP transistor in series with resistor 475 and an NPN transistor in a Darlington connection with the PNP transistor. In addition, there is a need to control this Darlington circuit during power up and possible brown-outs. These additional elements further complicate the power supply and render the power circuit 400 too expensive for consumer type applications.
As such, there is a need for a power circuit that is capable of generating a low voltage power supply that is significantly less than one watt for an electronic device in a standby condition.